The use of specially designed organophosphorus compounds as co-catalysts for transition metal catalysis has become very important industrially for the manufacture of many chemical products. Many of these phosphorus compounds, normally referred to as “ligands”, have two or more phosphorus atoms attached to the hydrocarbon backbone in an arrangement that constitutes the unique structure of a given ligand. These can be used for hydrogenation, polymerization and carbonylation co-catalysts. The use of bidentate ligands as co-catalysts for the low pressure rhodium hydroformylation of olefins is of particularly high commercial importance. Bidentate refers to ligands that have two coordinating phosphorus sites.
There are several different classes of bidentate phosphorus-containing ligands available. Bidentate diphosphite ligands have been disclosed in U.S. Pat. No. 4,885,401 as co-catalysts with Rh in the hydroformylation of propylene. This class of bidentate ligands use a substituted bi-naphthyl or bi-phenyl backbone with oxygen linkages to the phosphite moieties connected to the 2,2′ or 6,6′-positions of the binapthyl or biphenyl backbones respectively. Other bidentate trioganophosphine ligands make use of a single aromatic ring. In triorganophosphine ligands, the phosphorus atoms are connected to three carbon bonds. In this case, two diphenylphosphino groups are attached to the single aromatic ring by methylene, or short alkyl chains such as ethyl. This class is represented in U.S. Pat. Nos. 4,824,977 and 4,960,949. Yet another class of bidentate ligand is represented by having two diphenylphosphino groups attached to each other by an all aliphatic carbon connection, as for example 1,4-bis(diphenylphosphino)butane, endo-cis-2,3-bis(diphenylphosphinomethyl)[2,2,1]tricycloheptane, and trans-1,2-bis(diphenylphosphinomethyl)-3,3-dimethylcylobutane. The latter two of these ligands were disclosed as comparative examples in U.S. Pat. No. 4,960,949.
Most particularly, bidentate ligands have been prepared from 2,2′-dimethyl-1,1′-biphenyl, where two diphenylphosphino groups are attached independently to the two methyl groups. Similar useful bidentate ligands were prepared from 2,2′-dimethyl-1,1′-binapthyl and 2,2′-dimethyl-1-phenyl-1′-napthyl. This class of ligand is particularly useful in the rhodium catalyzed hydroformylation of propylene to prepare butyraldehyde with a high selectivity to the linear isomer. These ligands are disclosed in U.S. Pat. Nos. 4,694,109, 4,755,624 and 4,760,194. The preparation of the hydrocarbon precursors of the ligand is carried out by the nickel catalyzed coupling reaction of one mole of Grignard reagent with one mole of a single-halogen containing aromatic ring. This is particularly useful when preparing symmetrical hydrocarbons such as the 2,2′-dimethyl-1,1′-biphenyl and 2,2′-dimethyl-1,1′-binaphthyl, where two halves containing equivalent aromatic rings are used. The preparation of 2,2′-dimethyl-1,1′-binaphthyl is described in the journal “Synthesis” p. 317 (1985). A complex, bis(triphenylphosphine)Ni(II)Cl2 is used in this reaction. The Grignard is reacted with 1-bromo-2-methylnaphthalene.
There is also a need to prepare ligand hydrocarbon precursors with a plurality of coupling units. The coupling reaction to prepare symmetrical biaryl-compounds has the advantage of not producing unwanted coupling co-products because the two halves are equivalent. Thus, there are many variations of symmetrical coupling reactions that can take advantage of “one pot” or “one stage” reactions. Several of these make use of nickel catalyzed reduction of chloro or bromo containing aromatic hydrocarbons using in-situ generation of either the Grignard intermediate, when using magnesium metal, or an unspecified organometallic when using zinc metal. The resulting materials then proceed to make the symmetrical biaryl coupling product. Some of these examples are found in U.S. Pat. Nos. 5,061,669, 4,939,309 and 4,912,271.
Two reactions have the advantage of being able to prepare unsymmetrical biaryl hydrocarbons. In U.S. Pat. No. 4,760,194, the cross coupling reaction of 1-bromomagnesium-2-methyl naphthalene with 2-bromotoluene in the presence of a triphenylphosphine nickel(II)Cl2 complex formed the unsymmetrical 2,2′-dimethyl-1-phenyl-1′-naphthyl coupling product. The reaction was carried out by the addition of the Grignard reactant to the solution of 1-bromo-2-methylnaphthalene containing the nickel catalyst.
The other reaction, generally known as the “Suzuki Reaction”, is a palladium catalyzed reaction of a halogen-containing aromatic compound with an aryl-boronic acid derivative in the presence of a mild base such as potassium carbonate. This very versatile reaction is used in many pharmaceutical applications. A review on Suzuki reaction is, A. Suzuki J. Organometallic Chemistry 576 (1999) pp. 147-168. The disadvantages of this reaction is the relatively high cost of the palladium catalyst required and the boronic acid derivatives. These materials have to be prepared from Grignard reagents and as such would incur an extra cost. The Suzuki reaction has been successful at polyarylation of di-, tri- and tetra-bromobenzene with phenylboronic acid, and 2-methylboronic acid. This reference makes use of a costly modifying ligand, cis,cis,cis-1,2,3,4-tetrakis(diphenylphosphinomethyl)cyclopentane to complete the reaction F. Berthiol et. al. J. Organometallic Chemistry 689 (2004) pp. 2786-2798.
In their efforts to prepare substituted biphenyls containing a single chloride, Y. Ikoma et. al. reacted ortho-tolylmagnesium bromide with 3-chloro- and 4-chloro-bromobenzene in the presence of unmodified NiCl2 catalyst Synthetic Communications 21 (3) pp. 481-487 (1991). Their premise for the reaction was that bromo-groups would be more reactive for coupling than the chloro group. The stoichiometric ratio of the two reactants was 1/1 of Grignard to chlorobromobenzene. The yields to the resulting 2-methyl-3′-chloro-1,1′-biphenyl was 86% and a terphenyl side product (where two Grignard reactants were coupled to the chloro-bromobenzene) was 7%. When using 4-chlorobromobenzene, the yield to the 2-methyl-4′-chloro-1,1′-biphenyl was 87% and 3% to the terphenyl side product. Ikoma noted that when a 1,2-bis(diphenylphosphino)ethane Ni(II)Cl2 (dppeNiCl2) catalyst was used under the same conditions, the yield to the mono-chloro biphenyl when using the 3-chlorobromobenzene, dropped to 75% and a slight increase in terphenyl yield to 9% was observed. In the case with dppeNiCl2 catalyst and the 4-chlorobromobenzene reaction, the yield to monochlorobiphenyl compound was 63% and the corresponding terphenyl side product was 9%. In their hands, an example was presented with bis(triphenylphosphine)Ni(II)Cl2 catalyst using the same conditions with the 4-chlorobromobenzene reactant, in this case, the yield to the desired monochlorobiphenyl dropped further to 59% with a slight increase to terphenyl coproduct of 12%. This information indicated that the triphenylphosphine modified catalyst, as practiced in their hands, was not that efficient for coupling two aryl-groups to an aromatic ring containing two halogen groups.
The capability to rationally prepare hydrocarbons with methyl groups in a specific arrangement permits the preparation of organophosphorus ligands that coordinate in a unique manner to a given transition metal catalyst. Thus, there remains a need to prepare ligand hydrocarbon precursors that have more than two coupling units and are adapted for creation of hydroformylation catalysts that are stable and highly selective for the production of oxo aldehyde products having a high linear/branched isomer ratio.